Conical reflector antenna

ABSTRACT

A conical reflector antenna is disclosed with a feed that is either a close approximation to a line source or a cylindrical structure which is electrically equivalent to a line source. Nearly complete control of its aperture illumination function is achieved by exciting various amounts of the line source or cylindrical feed. In addition to beamwidth control of a collimated beam that is linearly or circularly polarized, phasesensing monopulse operation is made possible by dividing a cylindrical feed structure into three equal sectors and incorporating conventional directional couplers for obtaining the difference between received signals from sectors on either side of a vertical axis for azimuth error, and the weighted difference between the sum of those signals and the signals from the third sector for an elevation error.

United States Patent inventors T. 0. Paine Administrator of the NationalAeronautics and Space Administration with respect to an invention of;Arthur F. Seaton, Palos Verdes Estates,

Calif. Appl. No. 848,776 Filed Aug. 11, 1969 Patented Nov. 23, 1971CONlCAL REFLECTOR ANTENNA 13 Claims, 13 Drawing Figs.

References Cited UNITED STATES PATENTS 8/1965 Schell 3,308,468 3/1967Hannan .i 343/779 FOREIGN PATENTS 801,886 9/1958 Great Britain 343/840Primary Examiner-Eli Lieberman Attorneys-G. T. McCoy, .1. H. Warden andMonte F. Mott ABSTRACT: A conical reflector antenna is disclosed with afeed that is either a close approximation to a line source or acylindrical structure which is electrically equivalent to a line source.Nearly complete control of its aperture illumination function isachieved by exciting various amounts of the line source or cylindricalfeed. in addition to beamwidth control of a collimated beam that islinearly or circularly polarized, phase-sensing monopulse operation ismade possible by dividing a cylindrical feed structure into three equalsectors and incorporating conventional directional couplers forobtaining the difference between received signals from sectors on eitherside of a vertical axis for azimuth error, and the weighted differencebetween the sum of those signals and the signals from the third sectorfor an elevation error PATENTEBNHV 2 MI I 3,623,114

SHEET 1 [IF 3 ARTHUR F. SEATON INVIZNTOR,

ATTORNEYS PATENTEDNUV 23 I971 SHEET 2 BF 3 F. SEATON INVIZN'I'OR.

% Wu/MN ATTORNEY ARTHUR PATENTEUuuv 23 1971 3,623.1 14

SHEET 3 [1F 3 3o 33 FIGT To P TOP V A H Zxy Exyz Av ARTHUR F. SEATONINVEN'IOR.

ATTORNEYS CONHCAL REFLECTOR ANTENNA ORIGIN OF THE INVENTION BACKGROUNDOF THE INVENTION This invention relates to a reflector-type antenna andmore particularly to the combination of a new reflector and means forfeeding the reflector to control its aperture illumination function, andfor phase-sensing monopulse operation, dividing the feed into threesectors.

In the past parabolic reflectors have been employed for antennas withconsiderable success, but proper focusing requires a point source feedso that control over the amplitude of the aperture illumination functionis severely limited. It would be desirable to have a reflector-typeantenna having all of the advantages of a parabolic reflector antennawith the additional advantage of control over its aperture illuminationfunction so that design for maximum gain of low sidelobes may beaccomplished as desired.

SUMMARY OF THE INVENTION In accordance with the present invention, areflector-type antenna isprovided with a conical reflector and a feedwhich is either a close approximation to a line source or some structurewhich is electrically equivalent to a line source such that any desireddistribution of the feed can be provided to reflect a beam out throughthe aperture of the cone in a predictable manner. Beamwidth is theneasily changed by exciting various amounts of the line source feed, witha wide beamwidth generated when only a small portion of the feed nearthe apex of the cone is excited and progressively narrower beamsgenerated as additional sections of the feed successively further fromthe apex are excited. The narrowest beam results when the full feed isexcited. For phase-sensing monopulse operation in a tracking system, thefeed is divided into three 120 sectors, two sectors A and B on oppositesides of a given axis and the third sector C centered about that axis. Adirectional coupler combines signals received by sectors A and B toprovide an error signal about that given axis in a difference arm. Thesum of the signals received by the sectors A and B are then combined ina second directional coupler to provide an error signal about a secondaxis perpendicular to the given axis, with proper attenuation of the sum(A+B) such that the difference [(A+B)C] is equal to zero when the targetis on the second axis.

DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates schematically a conicalreflector antenna with its reflector in section through the axisthereof.

FIG. 2 illustrates schematically a conical reflector antenna for ahalf-angle of 45 at the vertex thereof.

FIGS. 3a to 3d illustrates diagrammatically the illumination of aconical reflector aperture with a linearly polarized line source i'eedarrangement.

FIG. 4 illustrates schematically a front view of a conical reflectorantenna with a linearly polarized cylindrical source feed arrangement.

FIG. 5 illustrates a side view of the cylindrical source feedarrangement of FIG. 4 with the entire circumference of the cylinderthereof projected into the plane of the paper.

FIG. 6 illustrates schematically the arrangement of sections of acylindrical source feed in a conical reflector antenna, each sectionbeing adapted for selective excitation to switch beamwidth.

FIG. 7 is a diagram which illustrates the geometry of a cylindrical feedarrangement.

FIG. 8 illustrates in a perspective view a planar antenna array (withcrossed slots for circular polarization) adapted to be used iii acylindrical form for the feed arrangement of FIG. 7.

FIG. 9 illustrates a cylindrical source feed arrangement forphase-sensing monopulse operation.

FIG. 10 illustrates a network for the monopulse arrangement of FIG. 9.

DESCRIPTION OF THE PREFERRED EMBODIMENTS It has been discovered that aconical reflector will collimate a beam when excited by a properlyconstructed line source feed disposed along the axis of the reflector.By exciting more or less of the line source feed from the apex of thecone toward its aperture, the beamwidth may be readily changed fordifferent uses, such as for communications in a deep space mission whenit is desired to cover the earth between the 3 db. points at all rangeswithout losing too much signal strength at long ranges.

Other advantages will become apparent from the following detaileddescription, such as the inherently stronger structure of the conicalreflecting surface if solid, and the ease with which it can be made of alightweight material if flexible. It is especially applicable for largeantennas and may be built with a collapsible frame for convenientstorage when such a large antenna is not in use, particularly in a spacecraft.

Referring now to the drawings, a conical reflector 10 with a line sourcefeed is shown schematically in FIG. I to illustrate that a conicalreflector can be used as a directional antenna when the proper phase andpolarization constraints in the feed are satisfied. For a cone with avertex angle 2 1 and an axial line source feed, the phase constraint issatisfied and perfect collimation is achieved if rays emanate from theaxis at the angle 0,, when There is a general class of line source feedswhich may be employed with a conical reflector that will satisfy theconstraints of equation (I), namely feeds working in the broadside mode.However, except for the special case of a conical reflector with ahalf-angle of approximately 45", a broadside feed will require thevarious radiating elements (illustrated in FIG. I by dots, such as a dot12) to be excited with a progressive phase delay for a uniform phasefront illustrated by a dotted line 13.

For the special case of a conical reflector with a half-angle of 45, nointerelement progressive phase delay is required and a standing wavearray can be used for the feed. Accordingly, in order to describe thepresent invention with the simplest feed system, a linear feedcomprising radiative elements on the cone axis, such as elements 16 and17 and a reflector half-angle of 45 is assumed, as for a reflector 18 inFIG. 2. However, any half-angle and feed system which satisfies therestraints of equation (I) may be employed to practice the presentinvention if a uniform wave front is maintained by proper phase delay inthe excitation of the progressive elements of the line feed.

The polarization restraint is more difficult to satisfy than the phaserestraint. To introduce the problems in satisfying the polarizationrestraint, an ideal line source feed as schematically illustrated inFIGS. 1 and 2 will be assumed. The problem involves the creation aroundthe feed of a field that has a proper magnitude and polarization so thatwaves reflected from a conical surface will be oriented in the samedirection and will have the proper amplitude distribution. This ofcourse assumes the phase restraint is satisfied by either a 45 halfconeangle or proper interelement phase delay.

Assuming the 45 half-cone angle of FIG. 2, it can be shown that a feedline which radiates identically polarized waves in all directions willproduce a cylindrical main beam that has a null on the axis of the cone.It can be further shown that the polarization of the beam makes onecomplete revolution in space as a test probe is carried around the axisof the cone one turn.

The ideal feed for a cone reflector will produce two figureeightpatterns perpendicular to each other with polarizations as shown in FIG.3a which represents schematically a section through a 45 half-angleconical reflector 20 normal to its axis. The feed comprises four lineararrays of dipoles a,a', b and b excited through coaxial cables 21bunched in the center. Although this feed is linearly polarized tosimplify the explanation which follows, it should be understood that thefeed may be adapted for circular polarization as will be describedhereinafter.

The dipoles a and a are excited 180 out of phase and oriented parallelto the feed, as indicated by the dot and cross adjacent to each, and thedipoles b and b are perpendicular to the feed or cone axis and a smalldistance from it.

The dipoles a and a illuminate the conical reflector with a figure-eightpattern represented by two circles A and A. That figure-eight pattern ispolarized perpendicular to the paper with the E vector in the upper halfpointing into the paper, as indicated by the cross near the dipole a,while the E vector in the lower half points out of the paper asindicated by the dot near the dipole :1. Upon reflection of the fieldsin this A-A pattern from the conical reflector 20 at a locus of pointsof reflection represented by a circle 22, the vectors labeled E, on thecircle 22 will appear in the antenna aperture as shown.

The vectors E always point either directly toward or away from the feedaxis, and their amplitude anywhere on the circle 22 is determined by thedimension of the A-A' pattern in that direction. It is evident that aconsiderable amount of cross-polarized energy is present in the E,field, and that the aperture is only partly illuminated.

The dipoles band b are also fed out-of-phase insofar as the axis ofcircular symmetry is concerned and illuminate the reflector with afigure-eight pattern represented by two circles B and B. The addition ofthe 8-H pattern fills the remainder of the aperture and reduces thecross-polarized field in the following manner. The polarization of the8-8 pattern is in the plane of the paper and parallel to the locus ofpoints of reflection represented by the circle 22. Upon reflection ofthe fields in this pattern from the conical surface, the vectors labeledE on the circle 22 will appear in the aperture as shown.

On the principal axes the E, and E vectors are parallel and of equalamplitude as indicated by the four vectors on the x and y axes. in thequadrants, components of both the E, and E vectors exist and areeverywhere perpendicular to each other, The total signal, represented bythe vector E, in those areas, is the vector sum of the vectors E andE,,.

An expression for the sum is most easily achieved by breaking bothvectors 5,, and E into x and y components. Using the upper right handquadrant vectors as an example, as shown in FIGS. 3b and 3c, thefollowing equations may be written.

E =E cos i E =E 'sin i (2) and E E sin q E, =E cos l (3) It can be shownthat these expressions hold for all quadrants. Assume the followingconditions are imposed on the amplitudes of E, and E,, as a function ofD.

where and are the maximum values reached by E, and E,, respectively.

The equations for the x and y components can then be written as:

A E =E sin 4 cos A A E =E cos s cos =En cos 4,

If the further condition is imposed that A A n h m a! then E =5 sin cos4 E, =E, sin 41 (7) and E,,,=-E, cos 2 sin D E =E cos l (8) The totalcomponents in the two principal axes can now be summed: E ,=E., ,+E,,=E,, sin l cos I E cos l sin =O E,,=E,,;+E E (sin l -+cos l )=E, (10)Equation (10) shows that the fields assumed for the feed add to aconstant value with the polarization parallel to the y axis regardlessof the location in 9. Equation (9) shows that the cross-polarizedcomponent, E,, is everywhere reduced to zero, again completelyindependent of l Hence it can be stated that the conditions assumed andimposed on this feed produce the ideal feed for the conical reflector inwhich no energy is lost in cross-polarized lobes and the collimatedenergy is equally distributed on circles concentric with the axis.

The conditions for a linearly polarized beam are summarized forreference:

1. Two orthogonal figure-eight patterns must be generated by the feed.2. One pattern, polarized parallel to the feed, must obey the amplitudefunction:

A E.,=E s1n (11) sin I 3. The other pattern, polarized perpendicular tothe feed,

must obey the amplitude function.

E =E cos 35 (12) cos I r. The peak amplitudes of E, and 13,, must beequal.

A E E b 1 3 Physical realization of this ideal feed line would be verydifficult to achieve at any frequency due to the physical impossibilityof placing all radiating elements on the axis of the cone. At lowfrequencies, an array of very short dipoles transverse to the axis ofthe cone would produce the 8-8 pattern substantially as shown in FIGS.3a and 3d, but to produce the A-A' pattern, two additional dipoles wouldhave to be mounted parallel to the axis with an extremely small distancein terms of the wavelength between them. In addition, the dipoles a,a'must be fed 1 out of phase.

FlG. 3d shows diagrammatically in a perspective view the manner in whichthe cables 21 extend from the apex of the conical reflector 20 along theaxis thereof to feed dipoles a, a, b and b at the end, and similarlyarranged dipoles at various levels between the apex and the dipoles a,a, b and b in accordance with the invention. The dipoles a and a aredisposed parallel to each other at equal distances from the axis andparallel to the axis, while dipoles b and b are disposed parallel toeach other at equal distances from the axis and perpendicular to theaxis as shown in FIG. 3a,

Both the short dipoles b and b and the out-of-phase dipoles a and awould be extremely hard to feed efficiently. At microwave frequenciesthe use of short dipoles would be most difficult. Slotted waveguidearrays have proved to be much more practical. For example, a singlewaveguide having its longitudinal center line along the axis of the conecould be provided with four arrays of slots, two arrays of shunt slotson opposing broad walls. Such an arrangement would produce patternswhich approximate the ideal patterns shown in FIG. 3a. However, theresulting collimated beam would suffer some loss of gain fromcross-polarization components and phase errors. To approach the idealmore closely, the feed waveguide would have to be made to appearextremely small in terms of a wavelength, and loading of the waveguidewith a material of large dielectric constant would be required. Acompromise would be necessary between high performance and thedifficulties of working with a heavily loaded waveguide.

Before proceeding with a description of embodiments for a line sourcefeed which approaches the ideal, it should first be noted that circularpolarization can be achieved by any line source feed simply by replacingthe linear dipoles a, a, b and b of FIG. 3 with crossed dipoles. Thesedipoles would all be identical in that one of the crossed dipoles wouldbe parallel to the axis of the cone and the other perpendicular thereto.Each pair of crossed dipoles would then be fed in phase rotationstarting with 0 for one pair of crossed dipoles, and proceeding in onedirection with a phase shift in the feed to the remaining three crosseddipoles at 90 intervals.

It should also be noted that reference has been made to only one set ofdipoles for both linear and circular polarized feeds at one point alongthe axis of the cone reflector, but it should be understood that thepattern illustrated in FIG. 3a, or a similar one for a circularlypolarized feed, would be repeated at regular intervals along the axis ofthe conical reflector as schematically illustrated in FIGS. 1 and 2 andin the diagrammatic perspective view of FIG. 3d. It should also be notedthat the possibility exists of using waveguides with suitable slots forboth the linear and circularly polarized feed instead of dipoles.Bearnwidth change could then be readily accomplished by switchingdifferent levels of the feed along the axis of the conical reflector inand out, either individually by level or in groups by sections. Asmaller diameter of the cone near the apex would be illuminated for awide beam by exciting only the first one or two levels; as narrowerbeams are desired, successively larger diameters of the cone would beilluminated by exciting additional levels of radiating elements alongthe axis of the conical reflector.

The line source feed arrangement described with reference to FIGS. 3aand 3d will approach the ideal for satisfactory results in a relativelysmall antenna with just a few levels of radiating elements. For a largernumber of levels, a larger number of feed lines would be required, oneset of feed lines for each level of radiating elements to be separatelyexcited. Thus, even if coaxial lines are used, the multiplicity of feedlines disposed along the axis of the conical reflector would form such alarge bundle that the radiating elements at each level would have to bespaced too far from the axis, with resulting phase errors and loss ofenergy in cross-polarized lobes.

To minimize phase errors and loss of energy for large antennas, where aconical reflector has its greatest advantage over prior art parabolicreflectors, a cylindrical feed may be employed, instead of a line sourcefeed, in the form of a hollow cylinder which may consist of acylindrical reflecting surface with dipole elements mounted outside in amanner to be described with reference to FIGS. 4 through 7, orwaveguides curved into an annular form with radiating slots around theoutside, a desired number of waveguides then being stacked to form thehollow cylinder.

The cylinder may be made as large as necessary to accommodate thenumerous transmission lines required to run down its center for thepurpose of exciting the feed arrangement at various levels. The onlyrequirement is that a sufficiently large number of radiating elements(dipoles or waveguide slots) be placed around the cylinder to keep theinterelement spacing to approximately one-half wavelength or less sothat an effectively continuous illumination of the conical reflectorwill result.

Referring now to FIG. 4, one level of linearly polarized feed isprovided with a cylinder 30 and 12 equidistant dipoles, such as dipoles31- and 32, placed about the cylinder 30. In order that the linearpolarization of the signal be kept properly oriented after reflectionfrom the surface of a conical reflector 33, the dipoles are rotated witha progressive interelement rotation of 30 such that dipoles 31 and 34are parallel but l out-of-phase so that the vectorsof their radiatingenergy point along the axis of the conical reflector, one away from thevertex of the conical reflector, and the other toward the vertex asshown in FIG. 5 where the surface of the cylindrical reflector is shownin a flat plane in order that the relative positions of the dipoles maybe shown. Thus, the dipoles 31. and 34 correspond directly to thedipoles a, a in the ideal line source feed arrangement illustrated inFIG. 3. Dipoles 35 and 36 are perpendicular to the axis of the conicalreflector 33 and the dipoles 31 and 34 to correspond to the dipoles band b of the ideal line source feed of FIG. 3a. The pairs of dipolesprovided between the orthogonal dipoles in the cylindrical feedarrangement of FIGS. 4 and 5, and not present in the ideal line sourcefeed of FIG. 3a, effectively fill in the remainder of the aperturebetween the figure-eight patterns provided by the perpendicular dipoles.That fill-in minimizes phase errors and loss of energy which otherwisewould result due to the significant space between diametrical pairs ofthe perpendicular dipoles caused by the need for a cylindrical columnthrough which coaxial cables must run to each dipole at the variouslevels which are to be separately excited for beamwidth switching.

If each level of radiating elements is to be separately excited, thereflecting cylinder 30 will require a diameter suffciently large toaccommodate a maximum number of transmission lines, and of course thegreater the diameter of the reflecting cylinder 30 the larger the numberof radiating elements required at each level to maintain a spacingbetween elements of approximately one-half wavelength or less.

To minimize the number of transmission lines running through thecylindrical reflector 30, and still provide for selective beamswitching, the various levels may be grouped into sections W, X, Y and 2as illustrated in FIG. 6, preferably with progressively more levels ofradiating elements in each section such as l, 2, 4 and 8 levels in therespective sections W, X, Y and Z. With a 45 half-angle for the conicalreflector 33 and a 12-foot aperture, the cylindrical reflector 30 may be1 foot in diameter to accommodate 48 transmission lines. 12 for each offour sections having a cumulative total of elements. Each element isoriented for broadside radiation perpendicular to the axis of theconical reflector 33 since a 45 half-angle is selected. For any otherhalf-angle the elements must be oriented to radiate into the conicalreflector 33 in such a direction as to satisfy the conditions ofequation (I). An appropriate phase delay would then be required betweenlevels for a uniform phase front beyond the aperture of the conicalreflector 33 as noted hereinbefore with reference to FIG. 1. Such aphase shift may be provided by a phasing network or properly designedfeed line within each section.

Since the cylindrical reflector 30 has a sufiicient diameter toaccommodate a conventional horn antenna for an extremely broad beam ofapproximately 40, it may be advantageous to provide one as illustratedby horn antenna 40 in FIG. 6. For a narrow beam, of approximately 20,only the section W would be excited. For yet a narrower beam such as 10,both sections W and X would be excited and for a 5 beam section W, X, Yand Z would be excited. For the narrowest beam of approximately 2.5 allsections W, X, Y and Z would be excited. For each of the narrower beamsof 20, 10, 5 and 2.5, the collimated beam in the near field would have anull on the axis of the conical reflector 33 so that for each of thenarrower beams, it may be desirable to also excite the conical horn 40when any of the sections W, X, Y and Z are excited.

It should be noted that all the dipole elements illustrated in FIGS. 4and are fed in phase to produce the desired illumination of thereflector. To illustrate that the desired illumination is produced, thevector representing the field radiated by each dipole may be dividedinto two components, one parallel to the axis of the cylindricalreflector 33 (parallel to dipoles 31 and 34) and one orthogonal thereto(parallel to a diameter of the cylinder 30). A study of the vectordiagrams for each of the dipoles with reference to the axes justdescribed indicates a sinusoidal-cosinusoidal variation in amplitude ofthe components along the axes as a function of location on the cylinder30 when referenced to a particular point. By proper choice of thecoordinate system, the amplitude variation function of either componentmay be expressed as a function of sin b as can be deduced from FIG. 7where 1 is the angle between the position of the dipole 35 shown inFIGS. 4 and 5 to a particular location on the reflecting cylinder 30.

It is desired to obtain the far-field distribution at any farfield pointP as a function of D. The problem is more readily formulated if insteadof a discrete number of sources an infinite number of sources is allowedon the cylinder, each radiating uniformly into the half-space visiblefrom its location on the outside of the cylinder. This assumption shouldbe a very close approximation to the actual physical situation becausethe circumference is large in terms of a wavelength. Furthermore, inpractice the elements would have about onehalf-wavelength spacing, whichis sufiiciently close to be an approximation of a continuousdistribution. The far-field point P then will receive energy only fromthe elements visible to it, and the limits of integration become D -n12and l +1r/2. The phase of the signals reaching point P is determined byboth 1 and 1 since the elements lie on the circumference of a circle.The path length difference in radians is then seen from FIG. 7 to be[21ra/A cos( l 1 ,,)1 between the limits of integration. It

, is now possible to write the expression for the total field at P asthe integration of the contributions from those infinitesimal portionsof the continuous distribution visible between the limits of integrationas p a constant multiplier This expression is in the form assumed forthe ideal feed in the equatiohs( l l to l 3). A similar solution for theother component of field gives E,(l ,,)=E,,p cos b, (l6) This expressionis in the desired form and also indicates that the cylindrical feed, asstipulated and under the reasonable assumptions made in setting up thefar-field expression, will completely suppress the cross-polarizedcomponents and provide axially symmetric illumination of the conicalreflector.

If circular polarization is desired, the dipoles may be replaced bycrossed dipoles or crossed slots. Then, because a phase shift isequivalent to rotation of a circularly polarized element, a choice isavailable between feeding the elements in phase with a progressiveinterelement rotation and feeding them with a progressive interelementphase shifi with no rotation. In the case of a waveguide feed the use ofpve phase shift would allow more flexibility in the choice of slotpattern used to generate the circularly polarized wave and wouldgenerally be preferable. The total phase shift required in one triparound the feed is 360" (equal to the number of degrees of rotationrequired for the elements) or 30 per element for l2 elements. In generalthe interelement phase shift, 41,, will be tlt,,=360/n (17) where n isthe number of elements at each station on the cylinder.

A practical antenna consisting of a conical reflector and a circularlypolarized cylindrical feed may take the form shown in FIG. 6. As notedhereinbefore, five beamwidths can be generated at will by the excitationof different portions of the cylindrical feed. The 40 beam is generatedby the circularly polarized conical horn 40 set in the open end of thecylindrical feed. The 20 beam uses the conical horn plus the smallsection of feed W. The horn is fed through a line length equal to thetransit time of the wave reflected off the conical surface so that thetwo wave fronts will join at the top of the cylindrical feed and willalways be in phase regardless of the wavelength. The 10 beam is obtainedby excitation of the section X in addition to the section W and thehorn. The 5 and 2.5 beams are obtained by successive excitation of the Yand Z sections so that the entire aperture is finally illuminated.

Each of section W, X, Y and Z may consist of a circularly polarizedplanar array rolled up into a cylinder. The wave in the radiatingstructure would be traveling around the feed from a feed point in achosen direction as for a multilevel section 44 illustrated in FIG. 8.The various levels of the sections are fed in phase at points 45,46,...for wave travel in the direction indicated by an arrow. The feedat each point may be by cables, as noted hereinbefore, using acorporate-feed system. A progressive interelement phase shift could beobtained by making the guide wavelength A, slightly longer than wouldnormally be required for in-phase radiation. That is readilyaccomplished for a given free space wavelength A, by so providing thecharacteristics of the waveguide as to yield a 360ln interelement phaseshift to satisfy the polarization requirement of the conical reflectorantenna.

A conical reflector antenna may be given phase-sensing monopulsecapability for tracking by dividing a cylindrical feed structure intothree equal sectors X, Y and 2 about horizontal and vertical axes H andV, as shown in FIG. 9, and the incorporation of conventional directionalcouplers SI and 52 as shown in FIG. 10 for obtaining appropriate sumsand differences of separate signals x, y and 2 received by therespective sectors X, Y and Z from a conical reflector 50 (FIG. 9).Different feed lines connecting each of the sectors to the directionalcouplers are represented by the reference character x, y and z tocorrespond to the signal identification letter assigned to the sectorsA, B and C of the cylindrical feed structure in FIG. 9.

The directional couplers 51 and 52 (FIG. 10) provide horizontal andvertical error signals AH and AV according to the following equations:

where E, is an input to the coupler 52 from a sum arm of the coupler 51,and k is a constant introduced by the difi'erent attenuation factors of3 db. and 4.77 db. for the couplers 51 and 52, respectively, provided tocompensate for the fact that twothirds of the full aperture is used todevelop the signal 2,, and only one-third to develop the signal 2. Thehorizontal and vertical error signals may then be employed to redirectthe antenna through a conventional servomechanism to reduce those errorsignals to zero, thereby tracking the target.

Phase shifters 53 and 54 are provided in the feed lines for the y and zsignals to maintain proper phase of the signals being combined. Forexample, the phase shifter 53 may provide a phase shift while the phaseshifter 54 provides a +90 phase shift.

By dividing a cylindrical feed into four equal sectors and incorporatinga conventional sum and difference network, a conical antenna can provideeven more sensitivity in phasesensing monopulse operation. However, thatwould involve additional complexity in feed line and beam switchingnetworks for the four sectors.

It should be appreciated that the invention is in no sense dependentupon any particular fabrication technique and that modifications andvariations will occur to those skilled in the art. Accordingly, it isnot intended that the scope of the invention be determined by thedisclosed exemplary embodiments, but rather should be determined by thebreadth of the appended claims.

We claim:

1. In a directional antenna having a conical reflector and a feeddisposed inside said reflector along the axis thereof, an improvement inthe feed comprising a plurality of radiating elements disposed about theaxis of said reflector at various levels from a level near the apex ofsaid reflector to a level closer to the aperture of said reflector atthe end thereof opposite said apex, said elements being oriented todirect rays of radiant energy toward the inside surface of saidreflector at an angle with said axis of said conical reflector asmeasured from the'apex of said reflector, and said angle issubstantially equal to I80 less twice the half-angle of said reflectoras measured from said axis to the inside surface thereof, wherebycollimation of said rays emanating from the aperture of said reflectoris achieved, said elements at a given level being disposed about saidaxis in pairs, with elements of a given pair on opposite sides of saidaxis, said pairs being uniformly spaced about said axis and arranged toradiate with a progressive interelement rotation of 360/n, where n isthe number of said elements at said given level, thereby substantiallyreducing cross-polarization.

2. Apparatus as defined in claim 1 wherein all of said elements at saidgiven level are excited in phase.

3. Apparatus as defined in claim 2 wherein each element radiateslinearly polarized energy.

4. Apparatus as defined in claim 1 wherein each element radiatescircularly polarized energy, and elements of a given level are arrangedto radiate with a progressive interelement rotation of 360ln byorienting each element in a like manner and feeding all elements of saidgiven level in sequence around said axis with a progressive interelementphase shift of 360/n, where n is as before the number of said elementsat said given level.

5. Apparatus as defined in claim 1 wherein said feed system for a givenlevel comprises a waveguide curved into a cylindrical form having as itsaxis said axis of said reflector and having slots as elements disposedabout its axis for radiating energy 6. Apparatus as defined in claim 5wherein said slot elements are disposed about said axis of saidcylindrical form in pairs, with slot elements of a given pair onopposite sides of said axis, and said slot elements are uniformlyspaced.

7. Apparatus as defined in claim 6 wherein each of said slot elementsradiates circularly polarized energy, and all of said slot elements areoriented in like manner and fed with a progressive interelement phaseshift of 360ln, where n is the number of said elements at said givenlevel.

8. Apparatus as defined in claim 7 wherein said progressive interelementphase shift is achieved by making the guide wavelength of said waveguidelonger than required for inphase radiation.

9. Apparatus as defined in claim 8 wherein said levels are grouped insections for separate and selective excitation to provide a desired beamwidth of rays emanating from the aperture of said cone.

10. Apparatus as defined in claim 1 including means for combining saidelements into groups of adjacent elements to provide signalsproportional to radiant energy received by said elements in groups, and

means for deriving a first error signal proportional to the difierencebetween signals provided by first and second ones of said groups ofelements in adjacent sectors, whereby tracking said target in one planeis provided upon directing said antenna to reduce said first error si alto zero. 11. pparatus as defined in claim 10 including means forderiving a second error signal proportional to the difference betweensignals provided by third and fourth ones of said groups of elements inadjacent sectors, whereby tracking said target in a second plane whiletracking in said first plane is provided upon directing said antenna toreduce said second error to zero while directing it to reduce said firsterror signal to zero.

12. Apparatus as defined in claim ll wherein said first and second onesof said groups of elements receive radiant energy from equal sectors,said third one of said groups of elements consists of said first andsecond ones of said groups of elements combined, and said fourth one ofsaid groups of elements consists of all remaining ones of said elementsnot included in said third one of said groups.

13. Apparatus as defined in claim 12 wherein all sectors are of equalsize, and said first and second tracking planes are perpendicular.

1. In a directional antenna having a conical reflector and a feeddisposed inside said reflector along the axis thereof, an improvement inthe feed comprising a plurality of radiating elements disposed about theaxis of said reflector at various levels from a level near the apex ofsaid reflector to a level closer to the aperture of said reflector atthe end thereof opposite said apex, said elements being oriented todirect rays of radiant energy toward the inside surface of saidreflector at an angle phi with said axis of said conical reflector asmeasured from the apex of said reflector, and said angle issubstantially equal to 180* less twice the half-angle of said reflectoras measured from said axis to the inside surface thereof, wherebycollimation of said rays emanating from the aperture of said reflectoris achieved, said elements at a given level being disposed about saidaxis in pairs, with elements of a given pair on opposite sides of saidaxis, said pairs being uniformly spaced about said axis and arranged toradiate with a progressive interelement rotation of 360*/n, where n isthe number of said elements at said given level, thereby substantiallyreducing cross-polarization.
 2. Apparatus as defined in claim 1 whereinall of said elements at said given level are excited in phase. 3.Apparatus as defined in claim 2 wherein each element radiates linearlypolarized energy.
 4. Apparatus as defined in claim 1 wherein eachelement radiates circularly polarized energy, and elements of a givenlevel are arranged to radiate with a progressive interelement rotationof 360*/n by orienting each element in a like manner and feeding allelements of said given level in sequence around said axis with aprogressive interelement phase shift of 360*/n, where n is as before thenumber of said elements at said given level.
 5. Apparatus as defined inclaim 1 wherein said feed system for a given level comprises a waveguidecurved into a cylindrical form having as its axis said axis of saidreflector and having slots as elements disposed about its axis forradiating energy
 6. Apparatus as defined in claim 5 wherein said slotelements are disposed about said axis of said cylindrical form in pairs,with slot elements of a given pair on opposite sides of said axis, andsaid slot elements are uniformly spaced.
 7. Apparatus as defined inclaim 6 wherein each of said slot elements radiates circularly polarizedenergy, and all of said slot elements are oriented in like manner andfed with a progressive interelement phase shift of 360*/n, where n isthe number of said elements at said given level.
 8. Apparatus as definedin claim 7 wherein said progressive interelement phase shift is achievedby making the guide wavelength of said waveguide longer than requiredfor inphase radiation.
 9. Apparatus as defined in claim 8 wherein saidlevels are grouped in sections for separate and selective excitation toprovide a desired beam width of rays emanating from the aperture of saidcone.
 10. Apparatus as defined in claim 1 including means for combiningsaid elements into groups of adjacent elements to provide signalsproportional to radiant energy received by said elements in groups, andmeans for deriving a first error signal proportional to the differencebetween signals provided by first and second ones of said groups ofelements in adjacent sectors, whereby tracking said target in one planeis provided upon directing said antenna to reduce said first errorsignal to zero.
 11. Apparatus as defined in claim 10 including means forderiving a second error signal proportional to the difference betweensignals provided by third and fourth ones of said groups of elements inadjacent sectors, whereby tracking said target in a second plane whiletracking in said first plane is provided upon directing said antenna toreduce said second error to zero while directing it to reduce said firsterror signal to zero.
 12. Apparatus as defined in claim 11 wherein saidfirst and second ones of said groups of elements receive radiant energyfrom equal sectors, said third one of said groups of elements consistsof said first and second ones of said groups of elements combined, andsaid fourth one of said groups of elements consists of all remainingones of said elements not included in said third one of said groups. 13.Apparatus as defined in claim 12 wherein all sectors are of equal size,and said first and second tracking planes are perpendicular.